linux-kernel - Re: [RFC][PATCH 6/9] s390/time: Provide sched_clock

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Date:   Tue, 9 May 2023 08:42:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     bigeasy@...utronix.de, mark.rutland@....com, maz@...nel.org,
        catalin.marinas@....com, will@...nel.org, chenhuacai@...nel.org,
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Subject: Re: [RFC][PATCH 6/9] s390/time: Provide sched_clock_noinstr()

On Tue, May 09, 2023 at 08:13:59AM +0200, Heiko Carstens wrote:
> 
> 1;115;0cOn Mon, May 08, 2023 at 11:19:57PM +0200, Peter Zijlstra wrote:
> > With the intent to provide local_clock_noinstr(), a variant of
> > local_clock() that's safe to be called from noinstr code (with the
> > assumption that any such code will already be non-preemptible),
> > prepare for things by providing a noinstr sched_clock_noinstr()
> > function.
> > 
> > Specifically, preempt_enable_*() calls out to schedule(), which upsets
> > noinstr validation efforts.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/s390/include/asm/timex.h |   13 +++++++++----
> >  arch/s390/kernel/time.c       |   11 ++++++++++-
> >  2 files changed, 19 insertions(+), 5 deletions(-)
> ...
> > +static __always_inline unsigned long __get_tod_clock_monotonic(void)
> > +{
> > +	return get_tod_clock() - tod_clock_base.tod;
> > +}
> > +
> >  /**
> >   * get_clock_monotonic - returns current time in clock rate units
> >   *
> > @@ -216,7 +221,7 @@ static inline unsigned long get_tod_cloc
> >  	unsigned long tod;
> >  
> >  	preempt_disable_notrace();
> > -	tod = get_tod_clock() - tod_clock_base.tod;
> > +	tod = __get_tod_clock_monotonic();
> >  	preempt_enable_notrace();
> >  	return tod;
> >  }
> ...
> > +unsigned long long noinstr sched_clock_noinstr(void)
> > +{
> > +	return tod_to_ns(__get_tod_clock_monotonic());
> > +}
> > +
> >  /*
> >   * Scheduler clock - returns current time in nanosec units.
> >   */
> >  unsigned long long notrace sched_clock(void)
> >  {
> > -	return tod_to_ns(get_tod_clock_monotonic());
> > +	unsigned long long ns;
> > +	preempt_disable_notrace();
> > +	ns = tod_to_ns(get_tod_clock_monotonic());
> > +	preempt_enable_notrace();
> > +	return ns;
> >  }
> >  NOKPROBE_SYMBOL(sched_clock);
> 
> This disables preemption twice within sched_clock(). So this should either
> call __get_tod_clock_monotonic() instead, or the function could stay as it
> is, which I would prefer.

Duh. Will fix.